The present invention relates to methods and apparatus for measuring and controlling fluid flow rates, and in particular to an improved method and apparatus in which both the ascending and descending flanks of a heat pulse, injected into a fluid flow, are detected to control and regulate the frequency of the measuring functions.
Various systems are available for measuring fluid flow rates. One such system, for example, uses a vane positionable in and rotatable by a fluid flow. The rate of rotation of the vane is measured, and represents the flow rate of the fluid. However, because such systems are substantially mechanical in nature, they are subject to failure and tend to be inaccurate over a wide range of fluid flow rates.
To overcome the disadvantages of mechanical flow rate measuring devices, in another type of system a heat pulse generator introduces heat pulses into a fluid flow, and the pulses are detected by a sensor positioned downstream from the generator. The time of flight of the pulse between the generator and sensor is determined by detecting either the ascending or descending flank of the pulse, and represents the flow rate of the fluid. Although such systems are not readily subject to mechanical failure, a sufficient time interval must exist between successive heat pulses in order to ensure that the system has cooled down from a preceding heat pulse before a subsequent pulse is generated. Consequently, flow rate measurements are often subject to inaccuracies, either because the intervals between successive heat pulses are not sufficiently long to ensure system cool down, or because the intervals are overly long so that measurements are obtained less frequently than would otherwise be possible.
Among the various uses for fluid flow rate measuring systems are those in liquid two component systems, for example two component lacquer spray paint systems, which in recent years have gained increasing importance, particularly in the automobile industry. The reason for the popularity of two component lacquer systems lies in the superior qualities of the resulting coatings obtained, which quality can only be maintained if the components are metered with great accuracy. Lacquer spraying techniques used in most mass production processes require continuous, component flow rate measuring and dosing, and once adjusted the dosing rate for the system in question must always remain constant, since fluctuations can lead to serious loss of quality. Flow rate measuring systems for two component dosing apparatus known and used heretofore in spray paint systems do not meet these requirements.